fetch: return binary response bodies (base64) and response headers, for hosted file downloads #460

Hosted MCP servers that read files through the browser bridge cannot get a file's bytes back. workvivo-mcp hit this. Its documents live on a signed, CDN-style file host outside the server's allowed outbound hosts, so on a hosted deployment the only route to the bytes is the user's browser, and the bridge cannot carry them today.

What blocks it today

• The fetch verb decodes every response body with response.text(), in both the content-script and in-page paths. Binary bodies (PDF, docx/xlsx zips, images) come back corrupted: invalid UTF-8 becomes U+FFFD, which cannot be undone.
• InnerResponseFetchOk is { op, status, url, body: string }. No response headers are relayed, so a server cannot check Content-Type, Content-Length or Content-Disposition before it trusts a body.
• FetchInit has no field to ask for binary.
• Responses are capped at 5 MB (MAX_RESPONSE_BODY_BYTES).
• File hosts like this send no CORS headers, so even a binary-safe content-script fetch of the signed URL fails. It would have to run from the background/service worker with host_permissions, where CORS does not apply.

Proposal

• Request option: an opt-in responseType: 'base64' (or bodyEncoding) on FetchInit.
• Extension: read arrayBuffer() and return the body as base64, plus a relayed subset of headers: content-type, content-length, content-disposition.
• Size cap: a separate, larger cap for binary bodies, refusing early when content-length is over it.
• Where the fetch runs: from the background worker, so that a redirect to a no-CORS file host still works. Scope it to the hosts the pairing grants.
• Capability flag: add one to the protocol, so a server can tell whether the paired extension supports binary and refuse cleanly when it doesn't.

Who is waiting on this

• workvivo-mcp: hosted document reads currently refuse, with a clear "not supported on hosted servers yet".
• Other browser-bridge servers that download attachments would benefit too.